The present invention generally relates to well logging apparatus and, in a preferred embodiment thereof, more particularly relates to apparatus and methods for detecting joints at adjacent sections of a jointed tubular structure, such as a bore hole casing or production tubing, in a subterranean well.
It is often necessary in a completed subterranean well to precisely locate one or more of the joints that join the various longitudinal sections of a jointed tubular structure, such as a bore hole casing or production tubing. This need arises, for example, when it is necessary to precisely locate a tool such as a perforating gun, or another downhole structure such as a packer, within the jointed tubular structure relative to a well structure previously installed therein, such as a set of casing perforations. The tool to be set at a predetermined location within the tubular structure is typically lowered into the tubular structure on an elongated positioning member such as a slick line or a length of coil tubing, and the depth of the previously installed well structure may be readily found on the previously recorded joint and tally log for the well.
Given this readily available information it would seem at first glance to be a rather straightforward task to simply lower the tool into the tubular structure until the lowered depth odometer reading for the elongated positioning member was equal to the indicated joint log and tally depth for the previously installed well structure plus (or minus as the case may be) the desired offset distance between the tool to be set and the previously installed well structure. However, due to the considerable stretch of the elongated positioning member at substantial tool lowering depths, this approach is often characterized by an unacceptably low degree of tool positioning accuracy. Specifically, the odometer reading is not identical to the actual lowered depth of the tool.
For years the drilling and well services have employed various correlative joint logging techniques to indirectly overcome these tool positioning inaccuracies. One such technique entails the lowering of an electronic joint sensor into the jointed tubular structure on an electrically conductive wireline to detect a spaced series of joints in the general vicinity of the desired tool positioning location, and determine the lengths between the adjacent pairs of joints using the joint-to-joint odometer readings.
Using the fact that the joint-to-joint lengths in a jointed tubular structure such as a bore hole casing or production tubing tend to be detectably nonuniform, the series of determined joint-to-joint lengths are "matched" to an identical series of joint-to-joint lengths on the previously-recorded joint and tally log to identify precisely which series of joints have been detected. Using this correlated logging information, a precise correspondence between the odometer readings and the actual lowered depth of the joint sensor may be arrived at. In turn, this information may be used to determine an odometer reading precisely corresponding to the desired tool setting depth and the tool may then be lowered to this odometer reading with its precise positioning assured.
During this joint logging procedure, as the joint sensor is longitudinally moved, on the electrically conductive wireline, through the general tubular structure vicinity of interest it electromagnetically detects mass changes in the tubular structure indicative of the joints therein. Upon detecting a joint the sensor responsively generates an electrical signal pulse which is appropriately amplified and transmitted to the surface through the wireline. These electrical signal pulses are transmitted to and imprinted on an appropriate single pin strip chart recorder side-by-side with the corresponding lowered depth odometer readings. In addition to the post-completion correlative logging operations described above, wireline joint logging may also be used in initial logging procedures to establish, for example, the joint and tally log itself.
While wireline logging operations of this type are quite accurate, they also tend to be undesirably expensive, particularly in subsequent correlation logging operations for tool setting purposes, due to wireline footage charges and the crew and surface equipment typically required to carry out the wireline logging operations.
Heretofore the use of slick line (such as monofilament steel cable) in downhole tool setting procedures requiring correlative logging, although potentially less expensive than its wireline counterpart, has not been considered practical because of the inability to accurately correlate the location of the tool connected to the slick line with the location of previously installed well structures. Stated in another manner, while various sophisticated and relatively expensive equipment may be used in conjunction with a slick line to compensate for line stretch inaccuracies and precisely determine the depth of the tool in the tubular structure, conventional slick line tool setting systems have not had the capability of also performing the correlative logging functions necessary to precisely locate the tool relative to previously installed equipment in the jointed tubular structure.
In addition to electronic joint detectors of the type described above, various mechanical joint detectors have also been proposed. These types of detectors are typically provided with radially biased finger structures that resiliently enter the interior recesses of "cavity" type joints and provide detectable weight variations on the weight indicator at the surface when the finger structures snap into the joint recesses as the joint sensor is longitudinally moved through the tubular structure. The weight indicator variations constitute mechanical joint detection signals which may be used in the location of tools within a jointed tubular structure.
While mechanical joint detection apparatus of this general type does not require the transmission of electrical signals to the surface, and thus avoid the attendant expense of an electrically conductive wireline, it also has a decidedly undesirable limitation in that it can only detect joints of the cavity type. It cannot be used to detect "flush" type joints since joints of this type do not have interior recesses for the mechanical joint detector finger structures to snap into.
From the foregoing it can be readily seen that a need exists for improved tubular structure joint detection apparatus and associated methods that eliminate, or at least substantially minimize, the above-mentioned problems, limitations and disadvantages typically associated with conventional joint detection apparatus and methods. It is accordingly an object of the present invention to provide such improved apparatus and methods.